AccuWeather.com

By Heather Buchman, Meteorologist

Jul 21, 2011; 10:46 AM ET

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Children enjoy a cooling mist at the Henry Doorly Zoo in Omaha, Neb., Monday, July 18, 2011. A heat wave is smothering the Midwest with a heat index well over 100 in most places. Cooling centers are opening, and farmers are scrambling to prevent more livestock deaths. (AP Photo/Nati Harnik)

Another sweltering heat wave is gripping a large part of the U.S., with at least 17 states hitting the 100-degree mark Tuesday and Wednesday. More than 40 states have surpassed 90° this week.

"When all is said and done, with the number of days of extreme heat and humidity of the current heat wave, it may be more significant and impact a larger area than the deadly 1995 heat wave of July 12-15," according to Expert Senior Meteorologist Jim Andrews.

While it's already hot on the East Coast, the worst is on its way.

Areas along the Northeast's heavily-populated Interstate 95 corridor will have highs near or higher than 100° by the end of the week as the steamy air spreads eastward from the Plains.

What is so impressive about this heat wave is the humidity that is accompanying it. Even places such as Minneapolis, Minn., have had AccuWeather.com RealFeel® temperatures rising near 120°!

Minneapolis set an all-time record Tuesday when the dew point temperature, which provides an indication of how humid the air is, reached 82°.

"It's like getting slapped in the face with a wet sponge when you walk outside," said AccuWeather.com Senior Meteorologist Bernie Rayno. "The thing about this heat wave is you have the heat... you have the humidity... and you have no wind."

With no wind during the day and temperatures not even dropping below 80° at night in many places, there is no relief. Air conditioners are running 24 hours a day.

By the end of the week, people in New York City, Philadelphia and Washington, D.C., will face RealFeel® temperatures at or above 110°.

"Temperatures will topple 100° in the big cities," stated AccuWeather.com Senior Meteorologist Henry Margusity in reference to the Northeast. "It's the hottest weather these cities have had since last July."

This photo was posted on our AccuWeather.com Facebook page by Raina C., from Lakeville, Minn., Tuesday. Post your pictures of thermometers or how you're beating the heat on our Facebook page.

Factors behind the Heat Wave

The positioning of the jet stream (an area of maximum winds high above the ground) is the major player in heat waves. When there is what meteorologists call a large "ridge" in the jet stream, areas that lie under that ridge can experience heat waves.

"A ridge is essentially a big bubble of hot air," Margusity explained.

A ridge is currently located over the central U.S. and has been sending temperatures soaring above 100° from Texas to Montana and the Dakotas and into the upper 90s farther east early this week. Winds have generally been light across this region as well.

A cold front moving through the northern Plains will provide some cooling for Montana, the Dakotas and Minnesota, as well as generate stronger winds ahead of its arrival.